The disclosure relates generally to plunger lift systems and methods for lifting liquids from subterranean boreholes. More particularly, the disclosure relates to plunger pistons for lifting liquids in a production string to the surface.
Subterranean formations that produce gas often produce liquids such as hydrocarbon condensates (e.g., relatively light gravity oil) and water from the reservoir. Such liquids can result from the migration of liquids from the surrounding reservoir into the bottom of the wellbore, or result from the migration of vapors from the surrounding reservoir into the wellbore, which subsequently condense and fall back to the bottom of the wellbore. More specifically, as the vapors enter the wellbore and travel up the wellbore, their temperatures drop below the respective dew points and they transition from vapor phase into liquid condensate.
In some wells that produce both gas and liquid, the formation gas pressure and volumetric flow rate are sufficient to lift the liquids to the surface. In such “strong” wells, the accumulation of liquids in the bottom of the wellbore generally does not inhibit gas production as the liquids are continuously lifted to the surface by the flow of the production gas. However, in wells where the gas does not provide sufficient energy to lift liquids out of the well (i.e., the formation gas pressure and volumetric flow rate are not sufficient to lift liquids to the surface), the liquids accumulate in the wellbore. In particular, as the life of a gas well matures, reservoir pressures that drive gas production to surface slowly decline, resulting in lower production. At some point, the production gas velocities drop below the “Critical Velocity” (CV), which is the minimum velocity required to carry a droplet of water to the surface. As time progresses these droplets accumulate in the bottom of the wellbore. If a sufficient volume of liquids accumulate in the bottom of the wellbore, the well may eventually become “loaded” as the hydrostatic head of liquid imposes a pressure on the production zone sufficient to restrict and/or prevent the flow of gas from the production zone, at which point the well is “killed” or “shuts itself in.” As a result, it may become necessary to use artificial lift techniques to remove the accumulated liquid from the wellbore to restore and/or increase the flow of gas from the formation.
Plunger lift systems are one type of artificial lift technique that relies on a free piston that is dropped down the production string into the well. Often, the well is first shut-in at the wellhead to stop the upward flow of production fluids in the production string. The free piston is allowed to fall through the production string and any liquids therein to a bumper located at the lower end of the production string. The well is then opened at the wellhead, thereby allowing gas to flow into the production string below the piston. When the pressure below the piston, due to the influx of gas, is sufficient, the piston is pushed upward through the production string to the surface, thereby lifting the liquids and gases in the production string disposed above the piston to the surface. This process is generally repeated to continually remove liquids from the production string.